This invention relates to a method for filling an electrolyte solution into a lithium secondary battery; said method capable of filling electrolyte solution into a case and extracting an excessive electrolyte solution therefrom, and sealing easily a battery, thereby the simplification of fabrication process, the reduction in the production cost, and the improvement in compaction of energy density can be achieved, and a battery structure of the lithium secondary battery; said battery having a reduced current collection resistance from positive electrodes and negative electrodes, and a narrowed deviation in the fluctuation in the resistances among the tabs engaged in current collection as well, and having a simple structure as a battery so as to enable easier assembly of the battery and to effectuate the aforementioned method for filling an electrolyte solution into the case easily.
In recent years, the lithium secondary battery has been widely used as a power battery for handy electronic appliances such as personal handy phone systems, video tape recorders, notebook-type computers, or the like. Additionally, in the case of a lithium secondary battery, a single battery can generate a voltage of approximately 4 V, and this level of voltage is higher than that of conventional secondary batteries such as a lead battery, or the like, and its energy density is also high. Thus, much attention has been paid to it not only as a power source for the aforementioned handy electronic appliances, but also as a motor driving power source for an electric vehicle (EV) or a hybrid electric vehicle (HEV), of which penetration among the general public is being earnestly planned as a low-pollution vehicle due to the recent development in the environmental problems.
In a lithium secondary battery, in general, a lithium-transition metal compound oxide as a positive active material, a carbon material as a negative active material, and an organic electrolyte solution obtained by dissolving a Li-ion forming electrolyte in an organic solvent as an electrolyte solution are used. And, for an internal electrode body as a portion where battery reaction is carried out, various types are available.
For example, in a coin-shaped battery with a small capacity, a sandwiched-type internal electrode body in which a separator is sandwiched between a positive electrode and a negative electrode is used. Here, as the positive electrode and the negative electrode, those that are formed in a disk like shape, or in a coin like shape by subjecting positive material and negative material to press-forming processing or the like, respectively are suitably used.
As one example of preferable structures of internal electrode bodies to be used for a lithium secondary battery with a comparatively large capacity usable for an EV, or the like, as is shown in FIG. 18, there is given a wound-type internal electrode body 1 being formed by winding around the outer periphery of a hollow cylinder-shaped core 6 a positive electrode 2 having one or more tabs 5 for current collection and a negative electrode 3 having one or more tabs 5 for current collection, in such a manner that the positive electrode 2 and negative electrode 3 are not brought into direct contact with each other, by sandwiching a separator 4 between the positive electrode 2 and the negative electrode 3. Here, in general, the length of the core 6 is set equal to the width of the positive electrode 2 and that of the negative electrode 3. Incidentally, there is also proposed a battery using a laminate-type internal electrode body formed by laminating alternately via separators 4 a plurality of positive electrodes 2 and negative electrodes 3 having been prepared by cutting the above-mentioned positive and negative electrodes, respectively into those with small areas.
Now, in any case where any of the above-described structures is adopted as an internal electrode body, it is necessary to soak the internal electrode body in an electrolyte solution. Here, as an electrolyte solution, a non-aqueous electrolyte solution (hereinafter to be referred to as an “electrolyte solution”), which is obtained by dissolving a lithium electrolyte in an organic solvent, is used. In the case of a coin-shaped battery, for example, there is employed such a technique that a predetermined quantity of an electrolyte solution is injected by using a metering pump, or the like, under a reduced atmosphere and the battery case is sealed so as to fill the case with the electrolyte solution, after the internal electrode body is mounted inside a battery case. In addition, even in the case where a wound-type internal electrode body is used, a similar technique is used as long as a small capacity battery such as a common 18650 (with a diameter of 18 mmφ and a length of 65 mm) cylinder-type battery is produced. In such a method, an excessive amount of electrolyte solution that is not actually required is liable to be filled therein.
Since electrolyte solution is generally expensive, the percentage of battery costs attributable to electrolyte solution is not small. Nevertheless, in the case of those batteries having a small capacity, the reasons why the aforementioned method for filling an electrolyte solution is adopted are considered that:                the space where excessive electrolyte solution (hereinafter to be referred to as a “excessive electrolyte solution”) is filled in is small in the absolute value, the cost for the electrolyte solution used for filling such a small space is considered not to be so high since the internal electrode body does not occupy much space in the interior of the battery in a small capacity battery;        a desired battery performance is obtainable if a minimum required quantity of an electrolyte solution is filled in a case since the area of reaction in the battery is small; and        an introduction of a step for recovering excessive electrolyte solution results in raising production costs unintentionally, etc.        
On the contrary, in the case of a battery having a relatively large capacity (hereinafter to be referred to as a “large capacity battery”) to be applied to an EV, or the like, the size of a battery itself will necessarily become large. In such a case, the use of the wound-type internal electrode body 1 shown in FIG. 18 requires a larger space for housing the current collection tabs 5 at both ends or one end of the case for the battery. Additionally, since a hollow cylinder-shaped type core is generally used for the core 6, the absolute volume to be occupied by these spaces inside the case for the battery becomes large.
Accordingly, if an electrolyte solution is filled into a case for a large capacity battery by using a technique similar to that for the above-described small capacity battery, an expensive electrolyte solution is used not in an economic manner. This would bring about an increase in the production cost and a reduction in the energy density of the battery, as well. Furthermore, it is not preferable, from the viewpoint of durability, for metal members other than the internal electrode body, sealing members of the battery case, and the like, to be always in contact with the electrolyte solution since it causes often the leakage of the electrolyte solution, the corrosion of said members, or the like.
On the other hand, the electrolyte solution is required to fill in an amount sufficient to impregnate the internal electrode body properly even in the case of a large internal electrode body having a large battery area. And in the case where this is not fulfilled, not only the desired battery performance cannot be attained, but also the fluctuation in the performance of respective batteries will take place. Accordingly, in the case of a large capacity battery, it is preferable to impregnate the internal electrode body thoroughly in an excessive amount of an electrolyte solution under a reduced atmosphere, and thereafter the excessive electrolyte solution is removed.
Therefore, in a large capacity battery, if one wants to fill an electrolyte solution by employing a technique similar to that for a small capacity battery, the following steps would be given as an example:                as shown in FIG. 17, at first, a case for battery 65 with one end portion 61 having been sealed is disposed in a globe box or the like with the sealed end 61 being placed downward,        then an electrolyte solution transferred from another end portion 62 of the case which is open at the upper portion with a metering pump or the like is injected by using a nozzle 63 or the like after reducing the atmosphere of the globe box in such a manner that the electrolyte solution is injected intermittently until the liquid surface does not go down so as to subject the internal electrode body to the impregnation treatment with the electrolyte solution for a predetermined period of time,        the interior of the globe box or the like is purged with inert gas,        thereafter the excessive electrolyte solution is drained by putting the case for battery 65 upside down, and        finally the end portion 62 which has been left open is sealed.        
However, in the case of such a method that an electrolyte solution is supplied from the upper portion of the case for the battery, the impregnation of an electrolyte solution starts mainly from the upper portion of the internal electrode body under a reduced atmosphere. Therefore, bubbles generated in the lower portion of the internal electrode body will hardly be liberated form the upper portion of the case for the battery. Accordingly, it will require holding the resultant for a long period of time under reduced atmosphere. In this case, if an organic solvent being highly volatile is solely used for an electrolyte solution, the evaporation of the solvent will bring about a problem in that the density of electrolyte fluctuates from product to product. In addition, in the case where a highly volatile organic solvent is mixed with one or more other non-volatile solvent or the like for use, the predominant evaporation of the volatile organic solvent causes the deviation in mixing ratio from product to product. This would bring about a problem in that the density of the electrolyte fluctuates from product to product. Anyhow, in any one of these cases, the full extent of exertion of the performance of electrolyte solution cannot be expected.
Moreover, in the case of a large capacity battery, due to a big shape of the battery itself, the sealing of an open end of the case for the battery within the globe box or the like would bring about various problems. That is, an enlargement of the globe box or the like is required since a sealing device should be installed within the globe box or the like. Furthermore, the enlargement of the globe box results in the decrease in the degree of the reduction of the interior pressure thereof, the enlargement of the vacuum pump, and the mass consumption of purge gas or the like. Thus, it is not realistic.
Therefore, the present inventors have extensively studied, in particular, the simplification of a method for filling an electrolyte solution in the production of a large capacity battery. As a result, they reached the present invention to be described later. Moreover, various studies have been made at the same time so as to find out not only a battery structure suitable for using the method of filling an electrolyte solution according to the present invention, but also a battery structure capable of improving the battery performance and productivity even in the case where the method for filling an electrolyte solution according to the present invention is not used.
One of the problems to be solved is the reduction in current collection resistance from the internal electrode body and the reduction in difference in current collection resistance of each tab. A tab is connected directly with an external terminal of the battery, that is, directly with an electrode terminal to extract current out from the battery, or is connected with an internal terminal thereof, that is, a terminal to which the tabs are intermediately connected collectively inside the battery. Accordingly, in the case where the tabs are connected with the internal terminal, it is necessary that the internal terminal is made conductive to the external terminal to form a current path between the tabs and the external terminal.
As a method for forming the conductive state between the tabs and the external terminal, there is proposed, for example, in JP-A-9-92338, a lithium secondary battery 27 in which a series of flexible leads (equivalent to “tabs” in meaning) 37 is sandwiched between the electrode terminal 38 and the hold-down hardware 33, forming a warping shape as shown in FIG. 16; said leads 37 being welded to the electrode terminal 38 by laser beam. In this lithium secondary battery 27, the electrode terminal 38 is attached to a cap (ceiling plate) 29 by using a nut 34, and the cap 29 is provided with not only electrolyte solution injection opening 32 which is to be sealed with a blank cap 30 but also a pressure release valve 26.
However, in case of the lithium secondary battery 27 disclosed in the JP-A-9-92338, the leads 37 may be sandwiched with the hold-down hardware 33 at any position of the outer periphery of the electrode terminal 38; as a corollary, the leads 37 disposed in the inner periphery of the internal electrode body 35 become long, and, on the contrary, the leads 37 disposed in the outer periphery become short. In his case, since the quantity of current flow in each lead 37 is different due to the difference in resistance of each lead 37, depending upon its length, there is a fear that the uniformity in the battery reaction cannot be maintained when used as a battery for an EV which requires the frequent flow of a large current.
In addition, since the leads 37 may be attached to any position of the outer periphery of the electrode terminals 38 with laser welding, and the structure at the end portion of the battery is complicated and various parts are installed therein, as shown in FIG. 16, thus the work efficiency (productivity) of the battery assembly is considered to be not necessarily good.
Moreover, a battery 27 disclosed in JP-A-9-92338 has the configuration at both ends, as shown in FIG. 16. It is stated in the laid-open invention that the injection of electrolyte solution is carried out by injecting electrolyte solution from one end of the injection opening 32 for electrolyte solution, while keeping the interior of the battery 27 under a reduced pressure by deaerating from the other end of the injection opening 32 for electrolyte solution, and this step should be repeated several times. However, it is not advantageous to assemble a battery with the repetition of such steps several times. Moreover, it is not advantageous to provide both ends with the injection openings 32 for electrolyte solution which eventually will be sealed since the leakage of the electrolyte solution and the decrease in air tightness are liable to occur.
Furthermore, the battery disclosed in JP-A-9-92338 has been proposed to prevent damage to leads 37 under severe vibrations when the battery is used as for the battery for an EV. Therefore, it proposes to use a flexible material for lead 37. At the same time, it refers to the reduction in internal resistance by virtue of a broadened welded portion between the leads 37 and the electrode terminals 38 formed by laser welding, however, it is quite silent about the reduction in fluctuation in the resistance among respective leads 37.
Another problem is how to secure the durability against vibration during driving since the durability is an essential requirement in the case of a battery for an EV. For example, when the internal electrode body vibrates or moves inside the battery case, there is a fear that the electrode active materials coated on the positive electrode and the negative electrode are peeled, thereby the battery capacity is reduced. Furthermore, it is not preferable since there is a fear of formation of a short circuit between the positive electrode and the negative electrode due to the peeled electrode active materials. Moreover, the end surface of the internal electrode body is apt to be deformed from an initial plain shape into a shape such as spiral waves or the like due to vibration, and such a deformation of the internal electrode becomes a cause of an unfavorable uneven battery reaction.
Therefore, there is proposed, in JP-A-9-92241, a battery 28 having such a structure that, as shown in FIG. 15, an electrode pole 25 having its lower surface covered with insulator collar 39 is inserted into a hollow portion of a cylindrical core 31 around which an electrode spiral body 36 (equivalent to the internal electrode body 1) is formed, and said electrode pole 25 is fixed to a cap 29 with a nut 34. In addition, there is proposed, in JP-A-1-751-76, a battery structure in which an internal electrode body formed by inserting a bar-shaped insulating body into a portion formed by using a tentative core which was removed thereafter is housed in the battery case.
However, in the case of the electrode spiral body 36 proposed in JP-A-9-92241, the inner peripheral surface of the battery case 19 and the electrode pole 25 function only as a stopper so as to suppress the movement of the electrode spiral body 36 in the diameter direction. However, it does not suppress the movement in the diameter direction, and it has such a structure that the movement in the longitudinal direction of the electrode spiral body 36 takes places easily in the distance of the gap with the electrode pole 25. If the movement to the longitudinal direction of the electrode spiral body 36 takes place, the electrode spiral body 36 collides with the electrode pole 25, which would damage the leads 37 (equivalent to tabs 5) attached on the end surfaces of the electrode spiral body 36. Moreover, it is considered that it is liable to receive such damage that the electrode active material is peeled, etc. at the end portions of the electrode spiral body 36.
Furthermore, in case of the invention disclosed in JP-A-1-175176, it is not formed in such a structure that the movement in the longitudinal direction of the internal electrode body is suppressed. This is because the internal electrode body is fixed by pressure formed between a solid bar of an insulator inserted into the inner peripheral surface of the battery case, and the core of the internal electrode body. Thus, no positive attempt has been made hitherto so as to suppress the movement in the longitudinal direction since much attention has been given to the fixation of the internal electrode body in the diametrical direction.